1. Field of the Invention
This invention relates to gas turbine engine rotor blades and, more particularly, to blade tips having localized compressive residual stresses imparted by laser shock peening.
2. Description of Related Art
Gas turbine engines such as aircraft gas turbine engine rotors often are designed so that during engine operation, the tip portion of the rotating blades rubs a stationary seal disposed within a shroud or casing circumscribing the blades so as to limit the leakage of working medium gases in the flow direction. These rotors operate at high rotational speeds that produce local high tensile and vibratory stress fields within blade tips and, particularly, make compressor and turbine blade tips susceptible to airfoil tip cracks followed by propagation and even failure (separation) of corners induced during the rubs. Vibrations may also be caused by vane wakes and inlet pressure distortions as well as other aerodynamic phenomena. Sensitivities and failure rates vary relative to the operating vibratory stresses, materials or degradation of properties and stress concentrations (due to tip wear and tear damage such as burrs, nicks, and tears) induced during rubs on the shroud or casing. A particularly troublesome combination occurs when all sensitivities combine to cause a failure in a relatively short time. Blades made from Titanium materials are notch sensitive, occasionally subject to relatively: severe rubs, and simultaneously may be subject to stress due to excitations of the blade in chordwise bending modes commonly referred to as "stripe modes", which has a potential for failing the blade under resonance in this mode. This can lead to separation and release of a corner of the blade. The mass size of this corner release is a function of the nodal pattern of the normal operating vibration mode. The corner release, in turn, impacts and damages downstream airfoils. The downstream damage often exceeds serviceable limits of the airfoils, therefore, necessitating engine removal for repairs and/or maintenance. Blades have been designed for easier maintenance and repair processes have been developed to repair worn and damaged blade tips. One short term solution is to cut back or crop the blade edges to raise the resonance frequency but this results in a loss of performance.
The burrs, nicks, and tears, hereinafter referred to as the tip damage, become the source of high stress concentrations or stress risers and may severely limit the life of the blades due to High Cycle Fatigue (HCF) from vibratory stresses discussed above. It is also expensive to refurbish and/or replace compressor and turbine blades and, therefore, any means to enhance the rotor capability and, in particular, to extend aircraft engine blade life is very desirable. The present invention is directed towards this end and provides a rotor blade with regions of deep compressive residual stresses imparted by laser shock peening along the blade's tip.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and, thereby, harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled "Method and Apparatus for Truing or Straightening Out of True Work Pieces". The present invention uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a work piece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser peening as understood in the art and as used herein means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". However, the prior art does not disclose blade tips of the type claimed by the present patent nor the methods of how to produce them. It is to this end that the present invention is directed.
The kinds of blade tip failures, even for engine designs in which vibration sensitivities are detected after numerous engines are already in service, can be eliminated by applying LSP to the blade tips. The compressive stresses from LSP can desensitize and neutralize the vibratory stresses and stress concentrations caused by the rub induced tip damage, therefore, preventing cracking and crack propagation.